Haem oxygenase delays programmed cell death in wheat aleurone layers by modulation of hydrogen peroxide metabolism

Endosperm cell death: roles and regulation in angiosperms

Double fertilization in angiosperms results in the formation of a second zygote, the fertilized endosperm. Unlike its embryo sibling, the endosperm is a transient structure that eventually undergoes developmentally controlled programmed cell death (PCD) at specific time points of seed development or germination. The nature of endosperm PCD exhibits a considerable diversity, both across different angiosperm taxa and within distinct endosperm tissues. In endosperm-less species, PCD might cause central cell degeneration as a mechanism preventing the formation of a fertilized endosperm. In most other angiosperms, embryo growth necessitates the elimination of surrounding endosperm cells. Nevertheless, complete elimination of the endosperm is rare and, in most cases, specific endosperm tissues persist. In mature seeds, these persisting cells may be dead, such as the starchy endosperm in cereals, or remain alive to die only during germination, like the cereal aleurone or the endosperm of castor beans. In this review, we explore current knowledge surrounding the cellular, molecular, and genetic aspects of endosperm PCD, and the influence environmental stresses have on PCD processes. Overall, this review provides an exhaustive overview of endosperm PCD processes in angiosperms, shedding light on its diverse mechanisms and its significance in seed development and seedling establishment.

Carbon Monoxide Alleviates Salt-Induced Oxidative Damage in Sorghum bicolor by Inducing the Expression of Proline Biosynthesis and Antioxidant Genes

Crop growth and yield are affected by salinity, which causes oxidative damage to plant cells. Plants respond to salinity by maintaining cellular osmotic balance, regulating ion transport, and enhancing the expression of stress-responsive genes, thereby inducing tolerance. As a byproduct of heme oxygenase (HO)-mediated degradation of heme, carbon monoxide (CO) regulates plant responses to salinity. This study investigated a CO-mediated salt stress tolerance mechanism in sorghum seedlings during germination. Sorghum seeds were germinated in the presence of 250 mM NaCl only, or in combination with a CO donor (1 and 1.5 μM hematin), HO inhibitor (5 and 10 μM zinc protoporphyrin IX; ZnPPIX), and hemoglobin (0.1 g/L Hb). Salt stress decreased the germination index (47.73%) and root length (74.31%), while hydrogen peroxide (H2O2) (193.5%), and proline (475%) contents increased. This increase correlated with induced HO (137.68%) activity and transcripts of ion-exchanger and antioxidant genes. Salt stress modified vascular bundle structure, increased metaxylem pit size (42.2%) and the Na+/K+ ratio (2.06) and altered primary and secondary metabolites. However, exogenous CO (1 μM hematin) increased the germination index (63.01%) and root length (150.59%), while H2O2 (21.94%) content decreased under salt stress. Carbon monoxide further increased proline (147.62%), restored the vascular bundle structure, decreased the metaxylem pit size (31.2%) and Na+/K+ ratio (1.46), and attenuated changes observed on primary and secondary metabolites under salt stress. Carbon monoxide increased HO activity (30.49%), protein content, and antioxidant gene transcripts. The alleviatory role of CO was abolished by Hb, whereas HO activity was slightly inhibited by ZnPPIX under salt stress. These results suggest that CO elicited salt stress tolerance by reducing oxidative damage through osmotic adjustment and by regulating the expression of HO1 and the ion exchanger and antioxidant transcripts.

Carbon monoxide promotes stomatal initiation by regulating the expression of two EPF genes in Arabidopsis cotyledons

The gaseous molecule carbon monoxide (CO) can freely pass through the cell membrane and participate in signal transduction in the cell to regulate physiological activities in plants. Here, we report that CO has a positive regulatory role in stomatal development. Exogenous CO donor CORM-2 [Tricarbonyldichlororuthenium (II) dimer] treatment resulted in an increase of stomatal index (SI) on the abaxial epidermis of cotyledons in wild-type, which can be reversed by the addition of the CO biosynthesis inhibitor ZnPPIX [Protoporphyrin IX zinc (II)]. Consistent with this result, mutation of the CO biosynthesis gene HY1 resulted in a decrease of SI in hy1-100 plants, while overexpression of HY1 led to an increase of SI. Further investigation revealed that CO acts upstream of SPCH and YDA in the stomatal development pathway, since the loss of function mutants spch-1 and yda-2 were insensitive to CORM-2. The expression of EPF2 was inhibited by CORM-2 treatment in wild type and is lower in hy1 than in wild-type plants. In contrast, the expression of STOMAGEN was promoted by CORM-2 treatment and is higher in HY1-overexpression lines. Loss of function mutants of both epf2 and stomagen are insensitive to CORM-2 treatment. These results indicated that CO positively regulates stomatal initiation and distribution by modulating the expression of EPF2 and STOMAGEN.

Improving the anthocyanin accumulation of hypocotyls in radish sprouts by hemin-induced NO

BACKGROUND

The health benefits of anthocyanins impel researchers and food producers to explorer new methods to increase anthocyanin contents in plant foods. Our previous studies revealed a positive role of nitric oxide (NO) in anthocyanin accumulation in radish (Raphanus sativus L.) sprouts. The application of hemin, an inducer of heme oxygenase-1 (HO-1), can effectively elevate NO production in vivo. Hemin treatment also improves plant growth and stress tolerance. This study is aimed to assess the effects of hemin treatment on anthocyanin production in radish sprouts, and to investigate whether NO signalling is involved in this process.

RESULTS

The application of hemin significantly up regulated the expressions of many anthocyanins biosynthesis related structure and regulatory genes, leading to increased anthocyanins accumulation in radish hypocotyls. Hemin treatment also raised NO contents in radish sprouts, probably through enhancing nitrate reductase (NR) activity and Nitric Oxide-Associated 1 (NOA1) expression. Comparing the effects of Zinc Protoporphyrin (ZnPP, HO-1 activity inhibitor), Sodium Nitroprusside (SNP, NO donor) and carboxy-PTIO (cPTIO, NO-scavenger) on anthocyanin and NO production, a positive role of NO signalling has been revealed in hemin-derived anthocyanin accumulation. A positive feedback loop between HO-1 and NO may be involved in regulating this process.

CONCLUSIONS

Hemin induced anthocyanin accumulation in radish sprouts through HO-1 and NO signalling network.

Transcriptomics View over the Germination Landscape in Biofortified Rice

Hidden hunger, or micronutrient deficiency, is a worldwide problem. Several approaches are employed to alleviate its effects (e.g., promoting diet diversity, use of dietary supplements, chemical fortification of processed food), and among these, biofortification is considered as one of the most cost-effective and highly sustainable. Rice is one of the best targets for biofortification since it is a staple food for almost half of the world’s population as a high-energy source but with low nutritional value. Multiple biofortified rice lines have been produced during the past decades, while few studies also reported modifications in germination behavior (in terms of enhanced or decreased germination percentage or speed). It is important to underline that rapid, uniform germination, and seedling establishment are essential prerequisites for crop productivity. Combining the two traits, biofortified, highly-nutritious seeds with improved germination behavior can be envisaged as a highly-desired target for rice breeding. To this purpose, information gathered from transcriptomics studies can reveal useful insights to unveil the molecular players governing both traits. The present review aims to provide an overview of transcriptomics studies applied at the crossroad between biofortification and seed germination, pointing out potential candidates for trait pyramiding.

Isorhynchophylline ameliorates paraquat-induced acute kidney injury by attenuating oxidative stress and mitochondrial damage via regulating toll-interacting expression

Isorhynchophylline (IRN) is an alkaloid with anti-inflammatory and anti-oxidative activities in cardiovascular and brain diseases, but its role in paraquat (PQ)-induced acute kidney injury (AKI) is yet unknown. The model of PQ-induced AKI in rats was established by intraperitoneal injection of PQ (25 mg/kg). We found that the tail vein injection of IRN (4 mg/kg) significantly increased the survival rate of PQ-intoxicated rats. IRN administration alleviated PQ-induced renal injury and renal dysfunction in rats, as evidenced by decreased apoptosis in renal cortex and reduced serum creatinine, serum BUN, and urine NGAL levels. Furthermore, IRN treatment improved the PQ-triggered oxidative stress in renal cortex by increasing the levels of anti-oxidant indicators (SOD activity, GSH/GSSG ratio, levels of Nrf-2, NQO-1, and HO-1 in renal cortex) and decreasing the levels of oxidative stress indexes (ROS and MDA levels in renal cortex). Interestingly, toll-interacting protein (Tollip), a negative regulator of interleukin 1 receptor associated kinase 1 (IRAK1) phosphorylation, was demonstrated to be increased by IRN injection in the renal cortex of PQ-intoxicated rats. In vitro experiments revealed that IRN protected renal tubular epithelial cells against PQ toxicity through suppressing oxidative stress and mitochondrial damage, and these protective effects were reversed by Tollip shRNA. Collectively, the present study demonstrated that IRN ameliorated PQ-induced AKI by attenuating oxidative stress and mitochondrial damage through upregulating Tollip, which provides new insights into the pathogenesis and treatment of PQ poisoning.

Hydrogen-rich water alleviates programmed cell death induced by GA in wheat aleurone layers by modulation of reactive oxygen species metabolism

Hydrogen gas (H₂) has been recently regarded as a novel gaseous signaling molecule that performs multiple functional roles in plant. Here, we demonstrate that hydrogen rich water (HRW)-an experimentally tractable reagent to assess the effects of the H₂ significantly delays wheat aleurone layer programmed cell death (PCD) induced by gibberellic acid (GA). Endogenous H₂ production exhibited lower level in aleurone layers under GA treatment, whereas the H₂ production was apparently increased under abscisic acid (ABA) treatment. HRW not only increased H₂ production but also delayed GA-induced PCD. We further observed that application of HRW substantially prevented the increases of hydrogen peroxide (H₂O₂) and superoxide anion radical (O₂^.-) triggered by GA. HRW also directly react with hydroxyl radical (·OH) to delay GA-induced PCD. Quantitative real-time PCR (qRT-PCR) and biochemical assays showed that HRW induced the transcripts and enzymatic activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) that metabolize reactive oxygen species (ROS); these increases coincided with the observed changes in O₂^.-, H₂O₂ and ·OH accumulation upon GA treatment. Our study therefore suggests that HRW-triggered alleviation of wheat aleurone layer PCD induced by GA results from a combination of H₂-mediated decreases of ROS levels, including O₂^.-, H₂O₂, and ·OH.

Interaction between endogenous H2O2 and OsVPE3 in the GA-induced PCD of rice aleurone layers

Endogenous hydrogen peroxide (H₂O₂) is involved in regulating the gibberellic acid-induced programmed cell death (PCD) of the aleurone layers by cooperating with OsVPE3 during rice seed germination. Preliminary experiments revealed that H₂O₂ produced by the NOX pathway is the key factor affecting rice germination. Histochemical analysis indicated that H₂O₂ is located in the aleurone layer. Both the H₂O₂ scavenger DMTU and the NOX inhibitor DPI decreased H₂O₂ content and significantly slowed down vacuolation in a dose-dependent manner. Interestingly, DMTU down-regulated the OsNOX8 transcript or DMTU and DPI decreased the intracellular H₂O₂ level, resulting in a delay of PCD. In contrast, GA and H₂O₂ up-regulated the OsNOX8 transcript and intracellular H₂O₂ level, leading to premature PCD, and the effects of GA and H₂O₂ were reversed by DMTU and DPI, respectively. These results showed that the imbalance of intracellular H₂O₂ levels leads to the delayed or premature PCD. Further experiments indicated that GA up-regulated the OsVPE3 transcript and VPE activity, and the effect was reversed by DPI. Furthermore, Ac-YVAD-CMK significantly blocked H₂O₂ accumulation, and DPI + Ac-YVAD-CMK had a more significant inhibitory effect compared with DPI alone, resulting in the delayed PCD, suggesting that OsVPE3 regulates PCD by promoting H₂O₂ generation. Meanwhile, DPI significantly inhibited the OsVPE3 transcript and VPE activity, and in turn delayed PCD occurrence, suggesting that the H₂O₂ produced by the NOX pathway may regulate PCD by up-regulating the OsVPE3 transcript. Thus, the endogenous H₂O₂ produced by the NOX pathway mediates the GA-induced PCD of rice aleurone layers by interacting with OsVPE3.

Cellular oxidative stress in programmed cell death: focusing on chloroplastic 1O2 and mitochondrial cytochrome-c release

The programmed cell death (PCD) occurs when the targeted cells have fulfilled their task or under conditions as oxidative stress generated by ROS species. Thus, plants have to deal with the singlet oxygen ¹O₂ produced in chloroplasts. ¹O₂ is unlikely to act as a primary retrograde signal owing to its high reactivity and short half-life. In addition to its high toxicity, the ¹O₂ generated under an excess or low excitation energy might also act as a highly versatile signal triggering chloroplast-to-nucleus retrograde signaling (ChNRS) and nuclear reprogramming or cell death. Molecular and biochemical studies with the flu mutant, which accumulates protochlorophyllide in the dark, demonstrated that chloroplastic ¹O₂-driven EXECUTER-1 (EX1) and EX2 proteins are involved in the ¹O₂-dependent response. Both EX1 and EX2 are necessary for full suppression of ¹O₂-induced gene expression. That is, EXECUTER proteolysis via the ATP-dependent zinc protease (FtsH) is an integral part of ¹O₂-triggered retrograde signaling. The existence of at least two independent ChNRS involving EX1 and β-cyclocitral, and dihydroactinidiolide and OXI1, respectively, seem clear. Besides, this update also focuses on plant PCD and its relation with mitochondrial cytochrome-c (Cytc) release to cytosol. Changes in the dynamics and morphology of mitochondria were shown during the onset of cell death. The mitochondrial damage and translocation of Cytc may be one of the major causes of PCD triggering. Together, this current overview illustrates the complexity of the cellular response to oxidative stress development. A puzzle with the majority of its pieces still not placed.

Haem oxygenase: A functionally diverse enzyme of photosynthetic organisms and its role in phytochrome chromophore biosynthesis, cellular signalling and defence mechanisms

Haem oxygenase (HO) is a universal enzyme that catalyses stereospecific cleavage of haem to BV IX α and liberates Fe⁺² ion and CO as by-product. Beside haem degradation, it has important functions in plants that include cellular defence, stomatal regulation, iron mobilization, phytochrome chromophore synthesis, and lateral root formation. Phytochromes are an extended family of photoreceptors with a molecular mass of 250 kDa and occur as a dimer made up of 2 equivalent subunits of 125 kDa each. Each subunit is made of two components: the chromophore, a light-capturing pigment molecule and the apoprotein. Biosynthesis of phytochrome (phy) chromophore includes the oxidative splitting of haem to biliverdin IX by an enzyme HO, which is the decisive step in the biosynthesis. In photosynthetic organisms, BVα is reduced to 3Z PΦB by a ferredoxin-dependent PΦB synthase that finally isomerised to PΦB. The synthesized PΦB assembles with the phytochrome apoprotein in the cytoplasm to generate holophytochrome. Thus, necessary for photomorphogenesis in plants, which has confirmed from the genetic studies, conducted on Arabidopsis thaliana and pea. Besides the phytochrome chromophore synthesis, the review also emphasises on the current advances conducted in plant HO implying its developmental and defensive role.

Exogenous Hydrogen Peroxide Contributes to Heme Oxygenase-1 Delaying Programmed Cell Death in Isolated Aleurone Layers of Rice Subjected to Drought Stress in a cGMP-Dependent Manner

Hydrogen peroxide (H2O2) is a reactive oxygen species (ROS) that plays a dual role in plant cells. Here, we discovered that drought (20% polyethylene glycol-6000, PEG)-triggered decreases of HO-1 transcript expression and HO activity. However, exogenous H2O2 contributed toward the increase in HO-1 gene expression and activity of the enzyme under drought stress. Meanwhile, the HO-1 inducer hematin could mimic the effects of the H2O2 scavengers ascorbic acid (AsA) and dimethylthiourea (DMTU) and the H2O2 synthesis inhibitor diphenyleneiodonium (DPI) for scavenging or diminishing drought-induced endogenous H2O2. Conversely, the zinc protoporphyrin IX (ZnPPIX), an HO-1-specific inhibitor, reversed the effects of hematin. We further analyzed the endogenous H2O2 levels and HO-1 transcript expression levels of aleurone layers treated with AsA, DMTU, and DPI in the presence of exogenous H2O2 under drought stress, respectively. The results showed that in aleurone layers subjected to drought stress, when the endogenous H2O2 level was inhibited, the effect of exogenous H2O2 on the induction of HO-1 was enhanced. Furthermore, exogenous H2O2-activated HO-1 effectively enhanced amylase activity. Application of 8-bromoguanosine 3',5'-cyclic guanosine monophosphate (8-Br-cGMP) (the membrane permeable cGMP analog) promoted the effect of exogenous H2O2-delayed PCD of aleurone layers in response to drought stress. More importantly, HO-1 delayed the programmed cell death (PCD) of aleurone layers by cooperating with nitric oxide (NO), and the delayed effect of NO on PCD was achieved via mediation by cGMP under drought stress. In short, in rice aleurone layers, exogenous H2O2 (as a signaling molecule) triggered HO-1 and delayed PCD via cGMP which possibly induced amylase activity under drought stress. In contrast, as a toxic by-product of cellular metabolism, the drought-generated H2O2 promoted cell death.

Novel insights into the effect of nitrogen on storage protein biosynthesis and protein body development in wheat caryopsis

Molecular and cytological mechanisms concerning the effects of nitrogen on wheat (Triticum aestivum L.) storage protein biosynthesis and protein body development remain largely elusive. We used transcriptome sequencing, proteomics techniques, and light microscopy to investigate these issues. In total, 2585 differentially expressed genes (DEGs) and 57 differentially expressed proteins (DEPs) were found 7 days after anthesis (DAA), and 2456 DEGs and 64 DEPs were detected 18 DAA after nitrogen treatment. Gene ontology terms related to protein biosynthesis processes enriched these numbers by 678 and 582 DEGs at 7 and 18 DAA, respectively. Further, 25 Kyoto Encyclopedia of Genes and Genomes pathways were involved in protein biosynthesis at both 7 and 18 DAA. DEPs related to storage protein biosynthesis contained gliadin and glutenin subunits, most of which were up-regulated after nitrogen treatment. Quantitative real-time PCR analysis indicated that some gliadin and glutenin subunit encoding genes were differentially expressed at 18 DAA. Structural observation revealed that wheat endosperm accumulated more and larger protein bodies after nitrogen treatment. Collectively, our findings suggest that nitrogen treatment enhances storage protein content, endosperm protein body quantity, and partial processing quality by altering the expression levels of certain genes involved in protein biosynthesis pathways and storage protein expression at the proteomics level.

Heme oxygenase 1 defects lead to reduced chlorophyll in Brassica napus

We previously described a Brassica napus chlorophyll-deficient mutant (ygl) with yellow-green seedling leaves and mapped the related gene, BnaC.YGL, to a 0.35 cM region. However, the molecular mechanisms involved in this chlorophyll defect are still unknown. In this study, the BnaC07.HO1 gene (equivalent to BnaC.YGL) was isolated by the candidate gene approach, and its function was confirmed by genetic complementation. Comparative sequencing analysis suggested that BnaC07.HO1 was lost in the mutant, while a long noncoding-RNA was inserted into the promoter of the homologous gene BnaA07.HO1. This insert was widely present in B. napus cultivars and down-regulated BnaA07.HO1 expression. BnaC07.HO1 was highly expressed in the seedling leaves and encoded heme oxygenase 1, which was localized in the chloroplast. Biochemical analysis showed that BnaC07.HO1 can catalyze heme conversion to form biliverdin IXα. RNA-seq analysis revealed that the loss of BnaC07.HO1 impaired tetrapyrrole metabolism, especially chlorophyll biosynthesis. According, the levels of chlorophyll intermediates were reduced in the ygl mutant. In addition, gene expression in multiple pathways was affected in ygl. These findings provide molecular evidences for the basis of the yellow-green leaf phenotype and further insights into the crucial role of HO1 in B. napus.

Carbon Monoxide Is Involved in Hydrogen Gas-Induced Adventitious Root Development in Cucumber under Simulated Drought Stress

Hydrogen gas (H2) and carbon monoxide (CO) are involved in plant growth and developmental processes and may induce plant tolerance to several stresses. However, the independent roles and interaction effect of H2 and CO in adventitious root development under drought conditions have still not received the needed research attention. We hypothesize that there exists crosstalk between H2 and CO during adventitious root development under drought stress. The results of our current study revealed that 50% (v/v) hydrogen-rich water (HRW), 500 μM Hemin (the CO donor) and 30% (w/v) CO aqueous solution apparently promoted the development of adventitious roots in cucumber explants (Cucumis Sativus L.) under drought stress. H2 and CO increased relative water content (RWC), leaf chlorophyll content (chlorophyll a, b, and a+b), and chlorophyll fluorescence parameters [photochemical efficiency of photosystem II (PSII), PSII actual photochemical efficiency and photochemical quench coefficient] under drought condition. When the CO scavenger hemoglobin (Hb) or zinc protoporphyrin IX (ZnPPIX) was added to HRW/CO aqueous solution, the positive effect of HRW/CO aqueous solution on RWC, leaf chlorophyll content, and chlorophyll fluorescence parameters were reversed. Additionally, superoxide dismutases, peroxidase, catalase, and ascorbate peroxidase was significantly increased in the explants treated with HRW and CO aqueous solution under drought stress, thus alleviating oxidative damage, as indicated by decreases in thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and superoxide radical (O2-) levels. H2 and CO also improved the levels of water soluble carbohydrate, total soluble protein, and proline content. However, the above CO/H2-mediated effects were reversed by CO scavenger Hb or CO specific synthetic inhibitor ZnPPIX. Therefore, CO may be involved in H2-induced adventitious rooting under drought stress and alleviate oxidative damage by enhancing RWC, leaf chlorophyll content, chlorophyll fluorescence parameters, metabolic constituent content, activating anti-oxidant enzymes and reducing TBARS, O2-, and H2O2 levels.

The relationship between vacuolation and initiation of PCD in rice (Oryza sativa) aleurone cells

Vacuole fusion is a necessary process for the establishment of a large central vacuole, which is the central location of various hydrolytic enzymes and other factors involved in death at the beginning of plant programmed cell death (PCD). In our report, the fusion of vacuoles has been presented in two ways: i) small vacuoles coalesce to form larger vacuoles through membrane fusion, and ii) larger vacuoles combine with small vacuoles when small vacuoles embed into larger vacuoles. Regardless of how fusion occurs, a large central vacuole is formed in rice (Oryza sativa) aleurone cells. Along with the development of vacuolation, the rupture of the large central vacuole leads to the loss of the intact plasma membrane and the degradation of the nucleus, resulting in cell death. Stabilizing or disrupting the structure of actin filaments (AFs) inhibits or promotes the fusion of vacuoles, which delays or induces PCD. In addition, the inhibitors of the vacuolar processing enzyme (VPE) and cathepsin B (CathB) block the occurrence of the large central vacuole and delay the progression of PCD in rice aleurone layers. Overall, our findings provide further evidence for the rupture of the large central vacuole triggering the PCD in aleruone layers.

Hydrogen Peroxide Is Involved in β-Cyclodextrin-hemin Complex-Induced Lateral Root Formation in Tomato Seedlings

Although previous results showed that β-cyclodextrin-hemin complex (β-CDH) could induce tomato lateral root (LR) formation, the corresponding downstream messengers are still not fully understood. In this report, similar to the inducing effects of exogenously applied hydrogen peroxide (H₂O₂), we discovered that β-CDH elicited RBOH1 transcript upregulation, endogenous H₂O₂ accumulation, and thereafter tomato LR development. Above responses were sensitive to dimethylthiourea (DMTU) and ascorbic acid (AsA), two membrane-permeable scavengers of H₂O₂, showing that accumulation of H₂O₂ and LR formation were significantly blocked. The test with diphenyleneiodonium (DPI; the inhibitor of NADPH oxidase) revealed that H₂O₂ mainly produced by NADPH oxidase, might be involved in LR formation triggered by β-CDH. qPCR combined with pharmacological and anatomical analyses showed that β-CDH-modulated several marker genes responsible for LR formation, such as CYCA3;1, CYCA2;1, CYCD3;1, and CDKA1 (four cell cycle regulatory genes), ARF7 and RSI-1 (two auxin signaling genes), LAX3 (an auxin influx carrier), IAA14 (encoding a member of the Aux/IAA protein family), PIN3 and PIN7 (two auxin efflux carriers), isocitrate dehydrogenase [NADP], NADH-cytochrome b₅ reductase 1, and L-ascorbate oxidase homolog genes (two reactive oxygen species-associated genes and one LR formation-related gene), were causally related to above H₂O₂ signaling. Particularly, representative proteins related to H₂O₂ metabolism and lateral rooting, were specifically induced in β-CDH-treated tomato seedlings. Overall, the results clearly suggested a vital role of H₂O₂ in the β-CDH-induced tomato LR formation, and β-CDH-elicited H₂O₂-related target proteins responsible for LR formation might be, at least partially, regulated at transcriptional and translational levels.

Carbon Monoxide as a Signaling Molecule in Plants

Carbon monoxide (CO), a gaseous molecule, has emerged as a signaling molecule in plants, due to its ability to trigger a series of physiological reactions. This article provides a brief update on the synthesis of CO, its physiological functions in plant growth and development, as well as its roles in abiotic stress tolerance such as drought, salt, ultraviolet radiation, and heavy metal stress. CO has positive effects on seed germination, root development, and stomatal closure. Also, CO can enhance plant abiotic stress resistance commonly through the enhancement of antioxidant defense system. Moreover, CO shows cross talk with other signaling molecules including NO, phytohormones (IAA, ABA, and GA) and other gas signaling molecules (H2S, H2, CH4).

Microarray: gateway to unravel the mystery of abiotic stresses in plants

Environmental factors, such as drought, salinity, extreme temperature, ozone poisoning, metal toxicity etc., significantly affect crops. To study these factors and to design a possible remedy, biological experimental data concerning these crops requires the quantification of gene expression and comparative analyses at high throughput level. Development of microarrays is the platform to study the differential expression profiling of the targeted genes. This technology can be applied to gene expression studies, ranging from individual genes to whole genome level. It is now possible to perform the quantification of the differential expression of genes on a glass slide in a single experiment. This review documents recently published reports on the use of microarrays for the identification of genes in different plant species playing their role in different cellular networks under abiotic stresses. The regulation pattern of differentially-expressed genes, individually or in group form, may help us to study different pathways and functions at the cellular and molecular level. These studies can provide us with a lot of useful information to unravel the mystery of abiotic stresses in important crop plants.

Plasma membrane H(+)-ATPase is involved in methyl jasmonate-induced root hair formation in lettuce (Lactuca sativa L.) seedlings

KEY MESSAGE

Our results show that methyl jasmonate induces plasma membrane H (+) -ATPase activity and subsequently influences the apoplastic pH of trichoblasts to maintain a cell wall pH environment appropriate for root hair development. Root hairs, which arise from root epidermal cells, are tubular structures that increase the efficiency of water absorption and nutrient uptake. Plant hormones are critical regulators of root hair development. In this study, we investigated the regulatory role of the plasma membrane (PM) H(+)-ATPase in methyl jasmonate (MeJA)-induced root hair formation. We found that MeJA had a pronounced effect on the promotion of root hair formation in lettuce seedlings, but that this effect was blocked by the PM H(+)-ATPase inhibitor vanadate. Furthermore, MeJA treatment increased PM H(+)-ATPase activity in parallel with H(+) efflux from the root tips of lettuce seedlings and rhizosphere acidification. Our results also showed that MeJA-induced root hair formation was accompanied by hydrogen peroxide accumulation. The apoplastic acidification acted in concert with reactive oxygen species to modulate root hair formation. Our results suggest that the effect of MeJA on root hair formation is mediated by modulation of PM H(+)-ATPase activity.

β-Cyclodextrin-hemin complex-induced lateral root formation in tomato: involvement of nitric oxide and heme oxygenase 1

β-Cyclodextrin-hemin complex-induced tomato lateral root formation was associated with nitric oxide and heme oxygenase 1 by modulating cell cycle regulatory genes. β-Cyclodextrin-hemin complex (β-CDH), a complex by combining β-cyclodextrin (β-CD) with hemin, a heme oxygenase 1 (HO1) inducer, was a trigger of cucumber adventitious root formation by enhancing HO1 gene expression. In this report, our results identified the previously unknown function of β-CDH in plants: the inducer of tomato lateral root (LR) formation. β-CDH-triggered LR formation is hemin-specific, since β-CD failed to induce LR development. Because nitric oxide (NO) is involved in LR formation, the correlation of β-CDH with NO and HO1 was investigated. Our analysis suggested that β-CDH induced an increase in endogenous NO production, followed by up-regulation of tomato HO1 gene and LR formation, all of which were mimicked by hemin and two NO-releasing compounds (SNP and GSNO). The induction of HO1 gene expression and LR formation triggered by β-CDH or hemin were significantly blocked by an inhibitor of HO1. Further results revealed that both β-CDH- and SNP-stimulated HO1 gene expression and thereafter LR formation were sensitive to the removal of NO with a potent NO scavenger, and the responses of SNP were significantly blocked by an inhibitor of HO1. Molecular evidence illustrated that representative cell cycle regulatory genes, including SlCDKA1, SlCYCA3;1, SlCYCA2;1, and SlCYCD3;1, were significantly up-regulated by β-CDH and SNP, but obviously blocked when seedlings were co-treated with the scavenger of NO or the inhibitor of HO1. In summary, our physiological and molecular evidence demonstrated that both NO and HO1 were involved in the β-CDH-induced LR formation with, at least partially, HO1 acting downstream of NO signaling.

Cobalt alleviates GA-induced programmed cell death in wheat aleurone layers via the regulation of H2O2 production and heme oxygenase-1 expression

Heme oxygenase-1 (HO-1) and hydrogen peroxide (H2O2) are key signaling molecules that are produced in response to various environmental stimuli. Here, we demonstrate that cobalt is able to delay gibberellic acid (GA)-induced programmed cell death (PCD) in wheat aleurone layers. A similar response was observed when samples were pretreated with carbon monoxide (CO) or bilirubin (BR), two end-products of HO catalysis. We further observed that increased HO-1 expression played a role in the cobalt-induced alleviation of PCD. The application of HO-1-specific inhibitor, zinc protoporphyrin-IX (ZnPPIX), substantially prevented the increases of HO-1 activity and the alleviation of PCD triggered by cobalt. The stimulation of HO-1 expression, and alleviation of PCD might be caused by the initial H2O2 production induced by cobalt. qRT-PCR and enzymatic assays revealed that cobalt-induced gene expression and the corresponding activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), three enzymes that metabolize reactive oxygen species, were consistent with the H2O2 accumulation during GA treatment. These cobalt responses were differentially blocked by co-treatment with ZnPPIX. We therefore suggest that HO-1 functions in the cobalt-triggered alleviation of PCD in wheat aleurone layers, which is also dependent on the enhancement of the activities of antioxidant enzymes.

Cobalt alleviates GA-induced programmed cell death in wheat aleurone layers via the regulation of H2O2 production and heme oxygenase-1 expression

Heme oxygenase-1 (HO-1) and hydrogen peroxide (H₂O₂) are key signaling molecules that are produced in response to various environmental stimuli. Here, we demonstrate that cobalt is able to delay gibberellic acid (GA)-induced programmed cell death (PCD) in wheat aleurone layers. A similar response was observed when samples were pretreated with carbon monoxide (CO) or bilirubin (BR), two end-products of HO catalysis. We further observed that increased HO-1 expression played a role in the cobalt-induced alleviation of PCD. The application of HO-1-specific inhibitor, zinc protoporphyrin-IX (ZnPPIX), substantially prevented the increases of HO-1 activity and the alleviation of PCD triggered by cobalt. The stimulation of HO-1 expression, and alleviation of PCD might be caused by the initial H₂O₂ production induced by cobalt. qRT-PCR and enzymatic assays revealed that cobalt-induced gene expression and the corresponding activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), three enzymes that metabolize reactive oxygen species, were consistent with the H₂O₂ accumulation during GA treatment. These cobalt responses were differentially blocked by co-treatment with ZnPPIX. We therefore suggest that HO-1 functions in the cobalt-triggered alleviation of PCD in wheat aleurone layers, which is also dependent on the enhancement of the activities of antioxidant enzymes.

Carbon monoxide regulates the expression of the wound-inducible gene ipomoelin through antioxidation and MAPK phosphorylation in sweet potato

Carbon monoxide (CO), one of the haem oxygenase (HO) products, plays important roles in plant development and stress adaptation. However, the function of CO involved in wounding responses is seldom studied. A wound-inducible gene, ipomoelin (IPO), of sweet potato (Ipomoea batatas cv. Tainung 57) was used as a target to study the regulation of CO in wounding responses. After wounding for 1h, the endogenous CO content and IbHO expression level were significantly reduced in leaves. IPO expression upon wounding was prohibited by the HO activator hemin, whereas the HO inhibitor zinc protoporphyrin IX elevated IPO expression. The IPO expression induced by wounding, H2O2, or methyl jasmonate was inhibited by CO. CO also affected the activities of ascorbate peroxidase, catalase, and peroxidase, and largely decreased H2O2 content in leaves. CO inhibited the extracellular signal-regulated kinase (ERK) phosphorylation induced by wounding. IbMAPK, the ERK of sweet potato, was identified by immunoblotting, and the interaction with its upstream activator, IbMEK1, was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Conclusively, wounding in leaves repressed IbHO expression and CO production, induced H2O2 generation and ERK phosphorylation, and then stimulated IPO expression.

Gibberellin-to-abscisic acid balances govern development and differentiation of the nucellar projection of barley grains

In cereal grains, the maternal nucellar projection (NP) constitutes the link to the filial organs, forming a transfer path for assimilates and signals towards the endosperm. At transition to the storage phase, the NP of barley (Hordeum vulgare) undergoes dynamic and regulated differentiation forming a characteristic pattern of proliferating, elongating, and disintegrating cells. Immunolocalization revealed that abscisic acid (ABA) is abundant in early non-elongated but not in differentiated NP cells. In the maternally affected shrunken-endosperm mutant seg8, NP cells did not elongate and ABA remained abundant. The amounts of the bioactive forms of gibberellins (GAs) as well as their biosynthetic precursors were strongly and transiently increased in wild-type caryopses during the transition and early storage phases. In seg8, this increase was delayed and less pronounced together with deregulated gene expression of specific ABA and GA biosynthetic genes. We concluded that differentiation of the barley NP is driven by a distinct and specific shift from lower to higher GA:ABA ratios and that the spatial-temporal change of GA:ABA balances is required to form the differentiation gradient, which is a prerequisite for ordered transfer processes through the NP. Deregulated ABA:GA balances in seg8 impair the differentiation of the NP and potentially compromise transfer of signals and assimilates, resulting in aberrant endosperm growth. These results highlight the impact of hormonal balances on the proper release of assimilates from maternal to filial organs and provide new insights into maternal effects on endosperm differentiation and growth of barley grains.

Regulation of potassium transport in plants under hostile conditions: implications for abiotic and biotic stress tolerance

Intracellular potassium homeostasis is a prerequisite for the optimal operation of plant metabolic machinery and plant's overall performance. It is controlled by K(+) uptake, efflux and intracellular and long-distance relocation, mediated by a large number of K(+) -selective and non-selective channels and transporters located at both plasma and vacuolar membranes. All abiotic and biotic stresses result in a significant disturbance to intracellular potassium homeostasis. In this work, we discuss molecular mechanisms and messengers mediating potassium transport and homeostasis focusing on four major environmental stresses: salinity, drought, flooding and biotic factors. We argue that cytosolic K(+) content may be considered as one of the 'master switches' enabling plant transition from the normal metabolism to 'hibernated state' during first hours after the stress exposure and then to a recovery phase. We show that all these stresses trigger substantial disturbance to K(+) homeostasis and provoke a feedback control on K(+) channels and transporters expression and post-translational regulation of their activity, optimizing K(+) absorption and usage, and, at the extreme end, assisting the programmed cell death. We discuss specific modes of regulation of the activity of K(+) channels and transporters by membrane voltage, intracellular Ca(2+) , reactive oxygen species, polyamines, phytohormones and gasotransmitters, and link this regulation with plant-adaptive responses to hostile environments.

In vitro evaluation of mitochondrial-chloroplast subcellular localization of heme oxygenase1 (HO1) in Glycine max

Heme oxygenase1 (HO1) catalyzes the degradation of heme in to biliverdin, carbon monoxide, and ferrous ions. Its role in higher plants has been found as an antioxidant and precursor of phytochrome synthesis. The present study focuses on subcellular localization of HO1 in leaves of soybean has been investigated. Most activity appeared to be located within chloroplast due to its role in phytochrome synthesis but mitochondria also share its localization. Mitochondrial location of HO1 might be on its inner membranous space due to its role in the synthesis of electron donor species which facilitates HO1 catalyzed reaction. Study reports the co-localization of HO1 in both chloroplast and mitochondria.

Winter wheat cells subjected to freezing temperature undergo death process with features of programmed cell death

Programmed cell death is a process defined as genetically regulated self-destruction or cell suicide. It can be activated by different internal and external factors, but few studies have investigated whether this process occurs under cold and freezing temperatures. In this study, a freezing treatment (-8 °C for 6 h) induced cell death with features of programmed cell death in suspension cultures of winter wheat (Triticum aestivum L.). This process occurred for 10 days after cold exposure. The death of cells in culture was slow and prolonged, and was accompanied by protoplast shrinkage, DNA fragmentation, and an increase in the level of reactive oxygen species. Other changes observed after the freezing treatment included an increase in the respiration rate, changes in mitochondrial transmembrane potential (∆Ψ m ), and the release of cytochrome c from mitochondria into the cytosol. These findings indicated that mitochondria are involved in the cell death process that occurs after a freezing treatment in cells of winter wheat.

The effects of redox controls mediated by glutathione peroxidases on root architecture in Arabidopsis thaliana

Glutathione peroxidases (GPXs) fulfil important functions in oxidative signalling and protect against the adverse effects of excessive oxidation. However, there has been no systematic characterization of the functions of the different GPX isoforms in plants. The roles of the different members of the Arabidopsis thaliana GPX gene (AtGPX) family were therefore investigated using gpx1, gpx2, gpx3, gpx4, gpx6, gpx7, and gpx8 T-DNA insertion mutant lines. The shoot phenotypes were largely similar in all genotypes, with small differences from the wild type observed only in the gpx2, gpx3, gpx7, and gpx8 mutants. In contrast, all the mutants showed altered root phenotypes compared with the wild type. The gpx1, gpx4, gpx6, gpx7, and gpx8 mutants had a significantly greater lateral root density (LRD) than the wild type. Conversely, the gpx2 and gpx3 mutants had significantly lower LRD values than the wild type. Auxin increased the LRD in all genotypes, but the effect of auxin was significantly greater in the gpx1, gpx4, and gpx7 mutants than in the wild type. The application of auxin increased GPX4 and GPX7 transcripts, but not GPX1 mRNAs in the roots of wild-type plants. The synthetic strigolactone GR24 and abscisic acid (ABA) decreased LRD to a similar extent in all genotypes, except gpx6, which showed increased sensitivity to ABA. These data not only demonstrate the importance of redox controls mediated by AtGPXs in the control of root architecture but they also show that the plastid-localized GPX1 and GPX7 isoforms are required for the hormone-mediated control of lateral root development.

Gibberellic acid-induced aleurone layers responding to heat shock or tunicamycin provide insight into the N-glycoproteome, protein secretion, and endoplasmic reticulum stress

The growing relevance of plants for the production of recombinant proteins makes understanding the secretory machinery, including the identification of glycosylation sites in secreted proteins, an important goal of plant proteomics. Barley (Hordeum vulgare) aleurone layers maintained in vitro respond to gibberellic acid by secreting an array of proteins and provide a unique system for the analysis of plant protein secretion. Perturbation of protein secretion in gibberellic acid-induced aleurone layers by two independent mechanisms, heat shock and tunicamycin treatment, demonstrated overlapping effects on both the intracellular and secreted proteomes. Proteins in a total of 22 and 178 two-dimensional gel spots changing in intensity in extracellular and intracellular fractions, respectively, were identified by mass spectrometry. Among these are proteins with key roles in protein processing and secretion, such as calreticulin, protein disulfide isomerase, proteasome subunits, and isopentenyl diphosphate isomerase. Sixteen heat shock proteins in 29 spots showed diverse responses to the treatments, with only a minority increasing in response to heat shock. The majority, all of which were small heat shock proteins, decreased in heat-shocked aleurone layers. Additionally, glycopeptide enrichment and N-glycosylation analysis identified 73 glycosylation sites in 65 aleurone layer proteins, with 53 of the glycoproteins found in extracellular fractions and 36 found in intracellular fractions. This represents major progress in characterization of the barley N-glycoproteome, since only four of these sites were previously described. Overall, these findings considerably advance knowledge of the plant protein secretion system in general and emphasize the versatility of the aleurone layer as a model system for studying plant protein secretion.

Hydrogen sulfide delays GA-triggered programmed cell death in wheat aleurone layers by the modulation of glutathione homeostasis and heme oxygenase-1 expression

Hydrogen sulfide (H2S) is considered as a cellular signaling intermediate in higher plants, but corresponding molecular mechanisms and signal transduction pathways in plant biology are still limited. In the present study, a combination of pharmacological and biochemical approaches was used to study the effect of H2S on the alleviation of GA-induced programmed cell death (PCD) in wheat aleurone cells. The results showed that in contrast with the responses of ABA, GA brought about a gradual decrease of l-cysteine desulfhydrase (LCD) activity and H2S production, and thereafter PCD occurred. Exogenous H2S donor sodium hydrosulfide (NaHS) not only effectively blocked the decrease of endogenous H2S release, but also alleviated GA-triggered PCD in wheat aleurone cells. These responses were sensitive to hypotaurine (HT), a H2S scavenger, suggesting that this effect of NaHS was in an H2S-dependent fashion. Further experiment confirmed that H2S, rather than other sodium- or sulphur-containing compounds derived from the decomposing of NaHS, was attributed to the rescuing response. Importantly, the reversing effect was associated with glutathione (GSH) because the NaHS triggered increases of endogenous GSH content and the ratio of GSH/oxidized GSH (GSSG) in GA-treated layers, and the NaHS-mediated alleviation of PCD was markedly eliminated by l-buthionine-sulfoximine (BSO, a selective inhibitor of GSH biosynthesis). The inducible effect of NaHS was also ascribed to the modulation of heme oxygenase-1 (HO-1), because the specific inhibitor of HO-1 zinc protoporphyrin IX (ZnPP) significantly suppressed the NaHS-related responses. By contrast, the above inhibitory effects were reversed partially when carbon monoxide (CO) aqueous solution or bilirubin (BR), two of the by-products of HO-1, was added, respectively. NaHS-triggered HO-1 gene expression in GA-treated layers was also confirmed. Together, the above results clearly suggested that the H2S-delayed PCD in GA-treated wheat aleurone cells was associated with the modulation of GSH homeostasis and HO-1 gene expression.

Programmed cell death (PCD): an essential process of cereal seed development and germination

The life cycle of cereal seeds can be divided into two phases, development and germination, separated by a quiescent period. Seed development and germination require the growth and differentiation of new tissues, but also the ordered disappearance of cells, which takes place by a process of programmed cell death (PCD). For this reason, cereal seeds have become excellent model systems for the study of developmental PCD in plants. At early stages of seed development, maternal tissues such as the nucellus, the pericarp, and the nucellar projections undergo a progressive degeneration by PCD, which allows the remobilization of their cellular contents for nourishing new filial tissues such as the embryo and the endosperm. At a later stage, during seed maturation, the endosperm undergoes PCD, but these cells remain intact in the mature grain and their contents will not be remobilized until germination. Thus, the only tissues that remain alive when seed development is completed are the embryo axis, the scutellum and the aleurone layer. In germinating seeds, both the scutellum and the aleurone layer play essential roles in producing the hydrolytic enzymes for the mobilization of the storage compounds of the starchy endosperm, which serve to support early seedling growth. Once this function is completed, scutellum and aleurone cells undergo PCD; their contents being used to support the growth of the germinated embryo. PCD occurs with tightly controlled spatial-temporal patterns allowing coordinated fluxes of nutrients between the different seed tissues. In this review, we will summarize the current knowledge of the tissues undergoing PCD in developing and germinating cereal seeds, focussing on the biochemical features of the process. The effect of hormones and redox regulation on PCD control will be discussed.

Chlorophyll deficiency in the maize elongated mesocotyl2 mutant is caused by a defective heme oxygenase and delaying grana stacking

BACKGROUND

Etiolated seedlings initiate grana stacking and chlorophyll biosynthesis in parallel with the first exposure to light, during which phytochromes play an important role. Functional phytochromes are biosynthesized separately for two components. One phytochrome is biosynthesized for apoprotein and the other is biosynthesized for the chromophore that includes heme oxygenase (HO).

METHODOLOGY/PRINCIPAL FINDING

We isolated a ho1 homolog by map-based cloning of a maize elongated mesocotyl2 (elm2) mutant. cDNA sequencing of the ho1 homolog in elm2 revealed a 31 bp deletion. De-etiolation responses to red and far-red light were disrupted in elm2 seedlings, with a pronounced elongation of the mesocotyl. The endogenous HO activity in the elm2 mutant decreased remarkably. Transgenic complementation further confirmed the dysfunction in the maize ho1 gene. Moreover, non-appressed thylakoids were specifically stacked at the seedling stage in the elm2 mutant.

CONCLUSION

The 31 bp deletion in the ho1 gene resulted in a decrease in endogenous HO activity and disrupted the de-etiolation responses to red and far-red light. The specific stacking of non-appressed thylakoids suggested that the chlorophyll biosynthesis regulated by HO1 is achieved by coordinating the heme level with the regulation of grana stacking.

Up-regulation of heme oxygenase-1 contributes to the amelioration of aluminum-induced oxidative stress in Medicago sativa

In this report, pharmacological, histochemical and molecular approaches were used to investigate the effect of heme oxygenase-1 (HO-1) up-regulation on the alleviation of aluminum (Al)-induced oxidative stress in Medicago sativa. Exposure of alfalfa to AlCl3 (0-100 μM) resulted in a dose-dependent inhibition of root elongation as well as the enhancement of thiobarbituric acid reactive substances (TBARS) content. 1 and 10 μM (in particular) Al(3+) increased alfalfa HO-1 transcript or its protein level, and HO activity in comparison with the decreased changes in 100 μM Al-treated samples. After recuperation, however, TBARS levels in 1 and 10 μM Al-treated alfalfa roots returned to control values, which were accompanied with the higher levels of HO activity. Subsequently, exogenous CO, a byproduct of HO-1, could substitute for the cytoprotective effects of the up-regulation of HO-1 in alfalfa plants upon Al stress, which was confirmed by the alleviation of TBARS and Al accumulation, as well as the histochemical analysis of lipid peroxidation and loss of plasma membrane integrity. Theses results indicated that endogenous CO generated via heme degradation by HO-1 could contribute in a critical manner to its protective effects. Additionally, the pretreatments of butylated hydroxytoluene (BHT) and hemin, an inducer of HO-1, exhibited the similar cytoprotective roles in the alleviation of oxidative stress, both of which were impaired by the potent inhibitor of HO-1, zinc protoporphyrin IX (ZnPP). However, the Al-induced inhibition of root elongation was not influenced by CO, BHT and hemin, respectively. Together, the present results showed up-regulation of HO-1 expression could act as a mechanism of cell protection against oxidative stress induced by Al treatment.

Cotton GhCKI disrupts normal male reproduction by delaying tapetum programmed cell death via inactivating starch synthase

Anther infertility under high temperature (HT) conditions is a critical factor contributing to yield loss in cotton (Gossypium hirsutum). Using large-scale expression profile sequencing, we studied the effect of HT on cotton anther development. Our analysis revealed that altered carbohydrate metabolism or disrupted tapetal programmed cell death (PCD) underlie anther sterility. Expression of the Gossypium hirsutum casein kinase I (GhCKI) gene, which encodes a homolog of casein kinase I (CKI), was induced in an HT-sensitive cotton line after exposure to HT. As mammalian homologs of GhCKI are involved in inactivation of glycogen synthase and the regulation of apoptosis, GhCKI may be considered a target gene for improving anther fertility under HT conditions. Our studies suggest that GhCKI exhibits starch synthase kinase activity, increases glucose content in early-stage buds and activates the accumulation of abscisic acid, thereby disturbing the balance of reactive oxygen species and eventually disrupting tapetal PCD, leading to anther abortion or indehiscence. These results indicate that GhCKI may be a key regulator of tapetal PCD and anther dehiscence, with the potential to facilitate regulation of HT tolerance in crops.

Roles of NIA/NR/NOA1-dependent nitric oxide production and HY1 expression in the modulation of Arabidopsis salt tolerance

Despite substantial evidence on the separate roles of Arabidopsis nitric oxide-associated 1 (NOA1)-associated nitric oxide (NO) production and haem oxygenase 1 (HY1) expression in salt tolerance, their integrative signalling pathway remains largely unknown. To fill this knowledge gap, the interaction network among nitrate reductase (NIA/NR)- and NOA1-dependent NO production and HY1 expression was studied at the genetic and molecular levels. Upon salinity stress, the majority of NO production was attributed to NIA/NR/NOA1. Further evidence confirmed that HY1 mutant hy1-100, nia1/2/noa1, and nia1/2/noa1/hy1-100 mutants exhibited progressive salt hypersensitivity, all of which were significantly rescued by three NO-releasing compounds. The salinity-tolerant phenotype and the stronger NO production in gain-of-function mutant of HY1 were also blocked by the NO synthetic inhibitor and scavenger. Although NO- or HY1-deficient mutants showed a compensatory mode of upregulation of HY1 or slightly increased NO production, respectively, during 2 d of salt treatment, downregulation of ZAT10/12-mediated antioxidant gene expression (cAPX1/2 and FSD1) was observed after 7 d of treatment. The hypersensitive phenotypes and stress-related genes expression profiles were differentially rescued or blocked by the application of NO- (in particular) or carbon monoxide (CO)-releasing compounds, showing a synergistic mode. Similar reciprocal responses were observed in the nia1/2/noa1/hy1-100 quadruple mutant, with the NO-releasing compounds exhibit the maximal rescuing responses. Overall, the findings present the combination of compensatory and synergistic modes, linking NIA/NR/NOA1-dependent NO production and HY1 expression in the modulation of plant salt tolerance.

Transcriptomic analysis of rice (Oryza sativa) endosperm using the RNA-Seq technique

The endosperm plays an important role in seed formation and germination, especially in rice (Oryza sativa). We used a high-throughput sequencing technique (RNA-Seq) to reveal the molecular mechanisms involved in rice endosperm development. Three cDNA libraries were taken from rice endosperm at 3, 6 and 10 days after pollination (DAP), which resulted in the detection of 21,596, 20,910 and 19,459 expressed gens, respectively. By ERANGE, we identified 10,371 differentially expressed genes (log(2)Ratio ≥1, FDR ≤0.001). The results were compared against three public databases (Gene Ontology, Kyoto Encyclopedia of Genes and Genomes and MapMan) in order to annotate the gene descriptions, associate them with gene ontology terms and to assign each to pathways. A large number of genes related to ribosomes, the spliceosome and oxidative phosphorylation were found to be expressed in the early and middle stages. Plant hormone, galactose metabolism and carbon fixation related genes showed a significant increase in expression at the middle stage, whereas genes for defense against disease or response to stress as well as genes for starch/sucrose metabolism were strongly expressed during the later stages of endosperm development. Interestingly, most metabolic pathways were down-regulated between 3 and 10 DAP except for those involved in the accumulation of material, such as starch/sucrose and protein metabolism. We also identified the expression of 1,118 putative transcription factor genes in endosperm development. The RNA-Seq results provide further systematic understanding of rice endosperm development at a fine scale and a foundation for future studies.

Haem oxygenase modifies salinity tolerance in Arabidopsis by controlling K⁺ retention via regulation of the plasma membrane H⁺-ATPase and by altering SOS1 transcript levels in roots

Reactive oxygen species (ROS) production is a common denominator in a variety of biotic and abiotic stresses, including salinity. In recent years, haem oxygenase (HO; EC 1.14.99.3) has been described as an important component of the antioxidant defence system in both mammalian and plant systems. Moreover, a recent report on Arabidopsis demonstrated that HO overexpression resulted in an enhanced salinity tolerance in this species. However, physiological mechanisms and downstream targets responsible for the observed salinity tolerance in these HO mutants remain elusive. To address this gap, ion transport characteristics (K(+) and H(+) fluxes and membrane potentials) and gene expression profiles in the roots of Arabidopsis thaliana HO-overexpressing (35S:HY1-1/2/3/4) and loss-of-function (hy-100, ho2, ho3, and ho4) mutants were compared during salinity stress. Upon acute salt stress, HO-overexpressing mutants retained more K(+) (less efflux), and exhibited better membrane potential regulation (maintained more negative potential) and higher H(+) efflux activity in root epidermis, compared with loss-of-function mutants. Pharmacological experiments suggested that high activity of the plasma membrane H(+)-ATPase in HO overexpressor mutants provided the proton-motive force required for membrane potential maintenance and, hence, better K(+) retention. The gene expression analysis after 12h and 24h of salt stress revealed high expression levels of H(+)-ATPases (AHA1/2/3) and Na(+)/H(+) antiporter [salt overly sensitive1 (SOS1)] transcripts in the plasma membrane of HO overexpressors. It is concluded that HO modifies salinity tolerance in Arabidopsis by controlling K(+) retention via regulation of the plasma membrane H(+)-ATPase and by altering SOS1 transcript levels in roots.

Heme oxygenase-1 is involved in nitric oxide- and cGMP-induced α-Amy2/54 gene expression in GA-treated wheat aleurone layers

Here, α-Amy2/54 gene expression was used as a molecular probe to investigate the interrelationship among nitric oxide (NO), cyclic GMP (cGMP), and heme oxygenase-1 (HO-1) in GA-treated wheat aleurone layers. The inducible expressions of α-Amy2/54 and α-amylase activity were respectively amplified by two NO-releasing compounds, sodium nitroprusside (SNP) and spermine NONOate, in a GA-dependent fashion. Similar responses were observed when an inducer of HO-1, hemin-or one of its catalytic products, carbon monoxide (CO) in aqueous solution-was respectively added. The SNP-induced responses, mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a cGMP derivative, were NO-dependent. This conclusion was supported by the fact that endogenous NO overproduction was rapidly induced by SNP, and thereafter induction of α-Amy2/54 gene expression and increased α-amylase activity were sensitive to the NO scavenger. We further observed that the above induction triggered by SNP and 8-Br-cGMP was partially prevented by zinc protoporphyrin IX (ZnPPIX), an inhibitor of HO-1. These blocking effects were clearly reversed by CO, confirming that the above response was HO-1-specific. Further analyses showed that both SNP and 8-Br-cGMP rapidly up-regulated HO-1 gene expression and increased HO activity, and SNP responses were sensitive to cPTIO and the guanylate cyclase inhibitor 6-anilino-5,8-quinolinedione (LY83583). Molecular evidence confirmed that GA-induced GAMYB and ABA-triggered PKABA1 transcripts were up-regulated or down-regulated by SNP, 8-Br-cGMP or CO cotreated with GA. Contrasting changes were observed when cPTIO, LY83583, or ZnPPIX was added. Together, our results suggested that HO-1 is involved in NO- and cGMP-induced α-Amy2/54 gene expression in GA-treated aleurone layers.

Disrupting the bimolecular binding of the haem-binding protein 5 (AtHBP5) to haem oxygenase 1 (HY1) leads to oxidative stress in Arabidopsis

The Arabidopsis thaliana L. SOUL/haem-binding proteins, AtHBPs belong to a family of five members. The Arabidopsis cytosolic AtHBP1 (At1g17100) and AtHBP2 (At2g37970) have been shown to bind porphyrins and metalloporphyrins including haem. In contrast to the cytosolic localization of these haem-binding proteins, AtHBP5 (At5g20140) encodes a protein with an N-terminal transit peptide that probably directs targeting to the chloroplast. In this report, it is shown that AtHBP5 binds haem and interacts with the haem oxygenase, HY1, in both yeast two-hybrid and BiFC assays. The expression of HY1 is repressed in the athbp5 T-DNA knockdown mutant and the accumulation of H(2)O(2) is observed in athbp5 seedlings that are treated with methyl jasmonate (MeJA), a ROS-producing stress hormone. In contrast, AtHBP5 over-expressing plants show a decreased accumulation of H(2)O(2) after MeJA treatment compared with the controls. It is proposed that the interaction between the HY1 and AtHBP5 proteins participate in an antioxidant pathway that might be mediated by reaction products of haem catabolism.

Disrupting the bimolecular binding of the haem-binding protein 5 (AtHBP5) to haem oxygenase 1 (HY1) leads to oxidative stress in Arabidopsis

The Arabidopsis thaliana L. SOUL/haem-binding proteins, AtHBPs belong to a family of five members. The Arabidopsis cytosolic AtHBP1 (At1g17100) and AtHBP2 (At2g37970) have been shown to bind porphyrins and metalloporphyrins including haem. In contrast to the cytosolic localization of these haem-binding proteins, AtHBP5 (At5g20140) encodes a protein with an N-terminal transit peptide that probably directs targeting to the chloroplast. In this report, it is shown that AtHBP5 binds haem and interacts with the haem oxygenase, HY1, in both yeast two-hybrid and BiFC assays. The expression of HY1 is repressed in the athbp5 T-DNA knockdown mutant and the accumulation of H(2)O(2) is observed in athbp5 seedlings that are treated with methyl jasmonate (MeJA), a ROS-producing stress hormone. In contrast, AtHBP5 over-expressing plants show a decreased accumulation of H(2)O(2) after MeJA treatment compared with the controls. It is proposed that the interaction between the HY1 and AtHBP5 proteins participate in an antioxidant pathway that might be mediated by reaction products of haem catabolism.

RNAi knockdown of rice SE5 gene is sensitive to the herbicide methyl viologen by the down-regulation of antioxidant defense

Plant heme oxygenase (HO) catalyzes the oxygenation of heme to biliverdin, carbon monoxide (CO), and free iron (Fe(2+))-and Arabidopsis and rice (Oryza sativa) HOs are involved in light signaling. Here, we identified that the rice PHOTOPERIOD SENSITIVITY 5 (SE5) gene, which encoded a putative HO with high similarity to HO-1 from Arabidopsis (HY1), exhibited HO activity, and localized in the chloroplasts. Rice RNAi mutants silenced for SE5 were generated and displayed early flowering under long-day conditions, consistent with phenotypes of the null mutation in SE5 gene reported previously (se5 and s73). The herbicide methyl viologen (MV), which produces reactive oxygen species (ROS), was applied to determine whether SE5 regulates oxidative stress response. Compared with wild-type, SE5 RNAi transgenic plants aggravated seedling growth inhibition, chlorophyll loss and ROS overproduction, and decreased the transcripts of some representative antioxidative genes. By contrast, administration of exogenous CO partially rescued corresponding MV hypersensitivity in the SE5 RNAi plants. Alleviation of seed germination inhibition, chlorophyll loss and ROS overproduction, as well as the induction of antioxidant defense were further observed when SE5 or HY1 was overexpressed in transgenic Arabidopsis plants, indicating that SE5 may be useful for molecular breeding designed to improve plant tolerance to oxidative stress.

Haem oxygenase-1 is involved in salicylic acid-induced alleviation of oxidative stress due to cadmium stress in Medicago sativa

This work examines the involvement of haem oxygenase-1 (HO-1) in salicylic acid (SA)-induced alleviation of oxidative stress as a result of cadmium (Cd) stress in alfalfa (Medicago sativa L.) seedling roots. CdCl(2) exposure caused severe growth inhibition and Cd accumulation, which were potentiated by pre-treatment with zinc protoporphyrin (ZnPPIX), a potent HO-1 inhibitor. Pre-treatment of plants with the HO-1 inducer haemin or SA, both of which could induce MsHO1 gene expression, significantly reduced the inhibition of growth and Cd accumulation. The alleviation effects were also evidenced by a decreased content of thiobarbituric acid-reactive substances (TBARS). The antioxidant behaviour was confirmed by histochemical staining for the detection of lipid peroxidation and the loss of plasma membrane integrity. Furthermore, haemin and SA pre-treatment modulated the activities of ascorbate peroxidase (APX), superoxide dismutase (SOD), and guaiacol peroxidase (POD), or their corresponding transcripts. Significant enhancement of the ratios of reduced/oxidized homoglutathione (hGSH), ascorbic acid (ASA)/dehydroascorbate (DHA), and NAD(P)H/NAD(P)(+), and expression of their metabolism genes was observed, consistent with a decreased reactive oxygen species (ROS) distribution in the root tips. These effects are specific for HO-1, since ZnPPIX blocked the above actions, and the aggravated effects triggered by SA plus ZnPPIX were differentially reversed when carbon monoxide (CO) or bilirubin (BR), two catalytic by-products of HO-1, was added. Together, the results suggest that HO-1 is involved in the SA-induced alleviation of Cd-triggered oxidative stress by re-establishing redox homeostasis.

Involvement of heme oxygenase-1 in β-cyclodextrin-hemin complex-induced cucumber adventitious rooting process

Our previous results showed that β-cyclodextrin-hemin complex (CDH) exhibited a vital protective role against cadmium-induced oxidative damage and toxicity in alfalfa seedling roots by the regulation of heme oxygenase-1 (HO-1) gene expression. In this report, we further test whether CDH exhibited the hormonal-like response. The application of CDH and an inducer of HO-1, hemin, were able to induce the up-regulation of cucumber HO-1 gene (CsHO1) expression and thereafter the promotion of adventitious rooting in cucumber explants. The effect is specific for HO-1 since the potent HO-1 inhibitor zinc protoporphyrin IX (ZnPP) blocked the above responses triggered by CDH, and the inhibitory effects were reversed further when 30% saturation of CO aqueous solution was added together. Further, molecular evidence showed that CDH triggered the increases of the HO-1-mediated target genes responsible for adventitious rooting, including one DnaJ-like gene (CsDNAJ-1) and two calcium-dependent protein kinase (CDPK) genes (CsCDPK1 and CsCDPK5), and were inhibited by ZnPP and reversed by CO. The calcium (Ca2+) chelator ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) and the Ca2+ channel blocker lanthanum chloride (LaCl3) not only compromised the induction of adventitious rooting induced by CDH but also decreased the transcripts of above three target genes. However, the application of ascorbic acid (AsA), a well-known antioxidant in plants, failed to exhibit similar inducible effect on adventitious root formation. In short, above results illustrated that the response of CDH in the induction of cucumber adventitious rooting might be through HO-1-dependent mechanism and calcium signaling.

KEY MESSAGE

Physiological, pharmacological and molecular evidence showed that β-cyclodextrin-hemin complex (CDH) was able to induce cucumber adventitious rooting through heme oxygenase-1 (HO-1)-dependent mechanism and calcium signaling.

The scutellum of germinated wheat grains undergoes programmed cell death: identification of an acidic nuclease involved in nucleus dismantling

Programmed cell death (PCD) is a crucial phenomenon in the life cycle of cereal grains. In germinating grains, the scutellum allows the transport of nutrients from the starchy endosperm to the growing embryo, and therefore it may be the last grain tissue to undergo PCD. Thus, the aim of this work was to analyse whether the scutellum of wheat grains undergoes PCD and to perform a morphological and biochemical analysis of this process. Scutellum cells of grains following germination showed a progressive increase of DNA fragmentation, and the TUNEL assay showed that PCD extended in an apical-to-basal gradient along the scutellum affecting epidermal and parenchymal cells. Electron-transmission microscopy revealed high cytoplasm vacuolation, altered mitochondria, and the presence of double-membrane structures, which might constitute symptoms of vacuolar cell death, whereas the nucleus appeared lobed and had an increased heterochromatin content as the most distinctive features. An acid- and Zn(2+)-dependent nucleolytic activity was identified in nuclear extracts of scutellum cells undergoing PCD. This nuclease was not detected in grains imbibed in the presence of abscisic acid, which inhibited germination. This nucleolytic activity promoted DNA fragmentation in vitro on nuclei isolated from healthy cells, thus suggesting a main role in nucleus dismantling during PCD.

Nitric oxide is involved in hemin-induced cucumber adventitious rooting process

Hemin, a heme oxygenase-1 (HO-1) inducer, was shown to exert numerous beneficial physiological functions in animals. Our previous study suggests that HO-1/carbon monoxide (CO) acts as a novel downstream signal system in the auxin-induced adventitious rooting. The objective of this study was to test whether nitric oxide (NO) is involved in hemin-induced cucumber adventitious rooting. Applications of hemin or CO aqueous solution to auxin-depleted cucumber explant induced up-regulation of cucumber HO-1 transcripts (CsHO1), NO production, and thereafter adventitious root formation, and some above responses were blocked by the combination treatment with two nitric oxide synthase (NOS)-like enzyme inhibitors N(G)-nitro-L-arginine methylester hydrochloride and N(G)-nitro-L-arginine, a HO-1 specific inhibitor zinc protoporphyrin IX, and a specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt. However, these blocking responses were not observed using tungstate, an inhibitor of nitrate reductase, another NO producing enzyme in plants. Furthermore, the guanylate cyclase inhibitors 1H-(1,2,4)-oxadiazole[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinedione reduced root development induced by hemin, whereas the cell-permeable cyclic guanosine monophosphate (cGMP) derivative 8-Br-cGMP reversed this effect. Together, our results indicated that at least in our experimental conditions, NO might operate downstream of hemin promoting adventitious root formation probably in a cGMP-dependent manner.